NP70 readonly Keyword

Question

Hi Guys

http://www.java2s.com/Code/CSharp/Language-Basics/Demonstratetheuseofreadonlyvariables.htm

I got this program from the above website. Reserved Keyword readonly has been used in the program. Program is producing similar output without readonly Keyword. I wish to know in this circumstance what is the purpose of using readonly Keyword.

Anyone knows please explain the reason.

Thank you

namespace nsReadOnly

{

using System;

public class ReadOnly

{

static double DegreeFactor = 1;

static double MilFactor = 0.05625;

static public void Main()

{

double degrees = 42;

// 1 degree = 17.77778 mils

double mils = degrees * 17.77778;

// 1 degree = 0.017453 radians

double radians = degrees * 0.017453;

clsArea InDegrees = new clsArea(DegreeFactor);

InDegrees.Angle = degrees;

InDegrees.Radius = 50;

Console.WriteLine("Area of circle is {0,0:F1}", InDegrees.Area);

// Radians are the default, so you can use the parameterless

// constructor

clsArea InRadians = new clsArea();

InRadians.Angle = radians;

InRadians.Radius = 50;

Console.WriteLine("Area of circle is {0,0:F1}", InRadians.Area);

clsArea InMils = new clsArea(MilFactor);

InMils.Angle = mils;

InMils.Radius = 50;

Console.WriteLine("Area of circle is {0,0:F1}", InMils.Area);

}

class clsArea

{

private const double pi = 3.14159;

private const double radian = 57.29578;

private readonly double m_Factor = 1;

private double m_Angle;

private double m_Radius;

public clsArea()

{

}

public clsArea(double factor)

{

m_Factor = factor / 57.29578;

}

public double Angle

{

get { return (m_Angle); }

set { m_Angle = value; }

}

public double Radius

{

get { return (m_Radius); }

set { m_Radius = value; }

}

public double Area

{

get

{

return (m_Radius * m_Radius * pi * m_Angle * m_Factor / (2 * pi));

}

/*

Area of circle is 916.3

Press any key to continue . . .

*/

Maha · Answer

Thank you Alan.

Alan · Answer

The significance of a readonly field can only be seen if you attempt to set it to a new value other than in its class's constructor. For example if you place this line in the Main() method an exception will be thrown: p1.y = 26; If 'y' hadn't been declared readonly then there would have been no exception. So readonly is a preventative language feature rather than a necessary one.

Maha · Answer

Program in the above website is producing similar result without readonly . Can a nyone explain the significance of the readonly keyword in this program . // cs_readonly_keyword.cs // Readonly fields using System; public class ReadOnlyTest { class SampleClass { public int x; // Initialize a readonly field public readonly int y = 25; public readonly int z; public SampleClass() { // Initialize a readonly instance field z = 24; } public SampleClass( int p1 int p2 int p3) { x = p1; y = p2; z = p3; } } static void Main () { SampleClass p1 = new SampleClass (11 21 32); // OK Console .WriteLine( p1: x={0} y={1} z={2}  p1.x p1.y p1.z); SampleClass p2 = new SampleClass (); p2.x = 55; // OK Console .WriteLine( p2: x={0} y={1} z={2}  p2.x p2.y p2.z); } } /* p1: x=11 y=21 z=32 p2: x=55 y=25 z=24 Press any key to continue . . . */

Maha · Answer

Thank you for your help Alan.

Alan · Answer

There's a full program example of readonly in the MSDN online docs: http://msdn2.microsoft.com/en-us/library/acdd6hb7(VS.90).aspx In my experience declaring an instance field as readonly is not done very often as most programmers prefer to use properties rather than expose fields directly and you can make a property readonly by just declaring a 'get'. However static readonly fields are used a fair bit because it's the only way you can declare a constant which cannot be evaluated until runtime and I thought Brandon's example brought out the difference between 'const' and 'readonly' very well.

Maha · Answer

Above one is a incomplete program it is difficult to understand novice programmer like me. Anyone knows produce a complete program please.

Brandon Lewis · Answer

Here you go: public class MyClass { private int myNum; public MyClass( int myNum) { this .myNum = myNum; } public int MyNum { get { return this .myNum; } } } public class ReadOnly { //The new keyword prevents this private const MyClass myClass = new MyClass (12); //So declare myClass as read only private readonly MyClass myClass = new MyClass (12); //Method to try to reassign to myClass public void ReassignMyClass() { //This will generate an error myClass = new MyClass (22); //But you can still access myClass's members int outNum = myClass.MyNum; } }

Maha · Answer

It is much appreciated if you support by an example either you can modify this program or give simple example where readonly is necessary.

Alan · Answer

The 'readonly' modifier is used when you want a field to always have the same value for a given instance. It can only be set at the same time it is declared or in the constructor. In the example you gave it made no difference whether m_Factor was declared as readonly or not because no attempt was made to set it again after it was declared and then in the constructor. If an attempt had been made to set it in another method then a compile time error would have occurred. It's also possible to have the 'static readonly' combination where you want a field to always have the same value for all instances. This can be used where you want to create a constant but can't use the 'const' modifier because the latter can only be set using a literal (eg: 1 2.3 'a' abc) or other constant expression. For example: public static readonly Color Blue = new Color(0 0 255); Here 'Blue' can't be declared as a const becuase of the presence of the 'new' expression. The essential difference is that the value of a 'const' is determined at compile time but the value of a 'static readonly' field is determined at runtime.